makefile: Invert check for lua vs lua5.1

Apparently ubuntu 13.x (?) installs lua5.1 by default for other packages
so checking for lua5.1 picks up the system installation by accident.

Instead now we check for lua first and then fall back to lua5.1 instead.

Makefile

@@ -64,7 +64,7 @@ endif
 #
 
 # some systems use lua5.1
-LUA=$(shell pkg-config lua5.1 && echo lua5.1 || echo lua)
+LUA=$(shell pkg-config lua && echo lua || echo lua5.1)
 MACOSX=$(shell uname -a | grep -qi darwin && echo 1 || echo 0)
 
 # which version of sdl do you want to ask pkgconfig for ?